We propose to study: 1) the composition of tubular fluid in regions of the kidney inaccessible to micropuncture; 2) the elemental composition of cells in the different segments of the nephron; 3) the concentration profile at high resolution for urea and non-urea solutes along the renal cortico-medullary axis and along the outer medulla transversal axis; 4) the permeability of the papillary epithelium to urea and the role of prostaglandins in the concentration mechanism and in the economy of water and salt; 5) the role of chloride permeability in the regulation of water reabsorption in the proximal tubule; 6) the relation of magnesium to potassium excretion, and the existence of humoral factor regulating magnesium excretion. The experiments will be done in rats by papillary perfusion, micropuncture recollection, and standing droplets in the proximal tubule. Besides electron probe analyses of Na, Cl, K, Mg, Ca, P and S in picoliter volumes, we will use two new methods: a fluorometric assay in picoliter volumes and electron probe analysis of kidney tissue samples maintained frozen hydrated at liquid nitrogen temperature.